In a conventional gas turbine engine comprising a compressor, combustor and turbine, both rotating turbine components such as blades, disks and retainers, and stationary turbine components such as vanes, shrouds and frames routinely require cooling due to heating thereof by hot combustion gases. Cooling of the turbine, especially the rotating components, is critical to the proper function and safe operation of the engine. Failure to adequately cool a turbine disk and its blading, for example, by providing cooling air deficient in supply pressure, volumetric flow rate or temperature margin, may be detrimental to the life and mechanical integrity of the turbine. Depending on the nature and extent of the cooling deficiency, the impact on engine operation may range from relatively benign blade tip distress, resulting in a reduction in engine power and useable blade life, to a rupture of a turbine disk, resulting in an unscheduled engine shutdown.
Balanced with the need to adequately cool the turbine is the desire for higher levels of engine operating efficiency which translate into lower fuel consumption and lower operating costs. Since turbine cooling air is typically drawn from one or more stages of the compressor and channelled by various means such as pipes, ducts and internal passageways to the desired components, such air is not available to be mixed with fuel, ignited in the combustor and undergo work extraction in the primary gas flowpath of the turbine. Total cooling flow bled from the compressor is therefore treated as a parasitic loss in the engine operating cycle, it being desirable to keep such losses to a minimum.
Prior art systems employ various schemes aimed at minimizing compressor bleed and concomitant cycle losses, for example, by attempting to control bleed source or cooling circuit parameters such as source pressure, pressure drop, flow rate or temperature. One type of system employs various active or passive means to modulate the volumetric flow rate of turbine cooling air at the bleed source, providing greater amounts when the need exists, for example as turbine temperature increases at high power throttle settings. Another type of system employs varying schemes of heat exchange, using engine lubrication system oil, for example, to cool the compressor bleed flow prior to delivery to the turbine. Such systems may reduce the amount of coolant flow required by delivering lower temperature air to the turbine. Alternatively, heat exchange systems may permit bleeding of the coolant from higher compression, higher temperature sources within the compressor which may better suit compressor operating efficiency or mechanical configuration. These and other known systems often add significant cost, weight and complexity to the engine. Further, such systems may consume precious volume within the engine or be mounted externally, thereby increasing the engine envelope in an attempt to make the components accessible for inevitable maintenance activity associated therewith. Finally, cooling system redundancy and/or fail-safe modes of operation are often implemented since the malfunction or failure of such systems is of concern, especially in light of the consequences resulting from inadequate turbine cooling.